CAD-CAM technologies have been established in the dental sector for some time and have taken the place of the traditional manual crafting of tooth replacements. However, the methods customary today for producing ceramic dental restoration elements by removing material have several disadvantages, which cannot be improved with reasonable expenditure from economic aspects by the current state of the art. In this connection, building-up methods of production that are known under the term “rapid prototyping” can be considered, in particular stereolithographic methods in which a newly applied layer of material is respectively polymerized in the desired form by position selective exposure, whereby the desired body is gradually produced by shaping in layers in its three-dimensional form, which results from the succession of the layers applied.
With respect to ceramic-filled polymers, WO 98/06560, which is hereby incorporated by reference, should be mentioned in particular. In this case, a ceramic slip is exposed by way of a dynamic mask (light modulator), whereby a three-dimensional body is intended to be gradually built up. In the case of the method described, the ceramic slip is exposed from above on a build platform. In the case of such exposure from above, after each exposure a new thin layer of material must be applied with the aid of a doctor blade (typically with a layer thickness which lies between 10 and 100 μm). When using materials of relatively high viscosity, as ceramic-filled resins are, it is only with difficulty, however, that such thin layers can be applied in a reproducible manner.
In the prior art, there are also known techniques, at least for photomonomers without ceramic filling, in which the exposure takes place from below through the bottom of a vat, which is formed by a transparent film, sheet or sheet with an elastomeric surface (for example of silicone or fluoroelastomer). Above the transparent film or sheet there is a build platform, which is held at a settable height above the film or sheet by a lifting mechanism. In the first exposure step, the photopolymer between the film and the build platform is polymerized in the desired form by exposure. When the build platform is raised, the polymerized first layer becomes detached from the film or sheet and liquid monomer flows into the space created. The object polymerized in layers is created by successive raising of the build platform and selective exposure of the monomer material that has flowed in. A device suitable for applying this method is described for example in DE 199 57 370 A1, which is hereby incorporated by reference. A similar procedure is described in DE 102 56 672 A1, which is hereby incorporated by reference, which however likewise relates to unfilled polymers.
In the processing of ceramic-filled photopolymers, the following problems arise in comparison with the processing of unfilled photopolymers:                The green strength of the polymerized objects is significantly lower (less than 10 MPa) than the strength of an unfilled polymer (typically about 20 to 60 MPa). As a result, the ceramic-filled photopolymer object can withstand significantly less mechanical loading (for example when the last-formed layer is detached from the sheet or film through which exposure was performed from below).        The high proportion of ceramic particles causes pronounced light diffusion, and the depth of penetration of the light that is used is significantly reduced. Associated with this is non-uniform polymerization in the z direction (direction of radiation) in the case of layer thicknesses of more than 20 μm. The small depth of penetration also makes it difficult to achieve reliable bonding of the first layer directly on the build platform. In the case of ceramic-filled monomer material, however, it cannot be ensured that the initial starting layer is sufficiently thin (for example less than 75 μm). Consequently, a reproducible bonding force on the build platform could not be ensured even with very long exposure of the first layer.        In comparison with unfilled photopolymers, ceramic-filled polymerizable materials are significantly more viscous. This imposes increased requirements on the exposure mechanism that is used. In particular, the time that is required for ceramic-filled photopolymer to flow in after raising of the build platform may be considerably longer. The raising and lowering of the build platform in a highly viscous photopolymer material also imposes increased requirements to avoid detrimental effects on the component.        On account of the high basic viscosity, ceramic-filled photopolymers are more sensitive to gelling by diffused light or ambient light. Even small light intensities are sufficient to raise the viscosity of the material above the permissible limit by the polymerization taking place.        